GPP was great.
Plus, Bob's new book is about writing interpreters!
It sounds like it may actually come in handy - I've ended up creating a number of solutions which came pretty close to being fully fledged domain specific languages. Sadly, despite nominally passing a CS compilers course about 20 years ago, my implementations have been horrifically incorrect. Maybe it's time to refresh my memory of how to actually do this.
I'd say it's worth the fun of writing your own alongside or after!